(1) Field of the Invention
The present invention relates to optical systems and particularly to an achromatic holographic imaging system. More specifically, this invention is directed to color correction in optical systems using diffractive elements. Accordingly, the general objects of the present invention are to provide novel and improved apparatus and methods of such character.
(2) Description of the Prior Art
Optical systems, for example telescopes, which incorporate conventional optical elements, such as mirrors and lenses, are well known in the art. Such systems are relatively large and heavy. It has been suggested, in the interest of reducing weight, to fabricate optical systems using diffractive optical elements such as holographic elements and/or zone plates.
When designing an optical systems to have high resolution it is essential that the primary element have a large aperture. In order to have this large aperture use conventional elements, the primary element must itself be relatively large. This requisite large size of the primary optical element hinders the design of some optical systems, telescopes for use in space for example, due to the difficulties of manufacturing and subsequent transporting large mirrors and lenses.
It has been determined that holographic optical elements, which possess the same aperture size as conventional optical elements, can be fabricated. Such holographic optical elements will be considerably smaller in size and weigh less than a conventional lens having comparable optical characteristics. This has led to the suggestion of fabricating holographic optical systems incorporating only holographic elements. For a discussion of such holographic systems, reference may be had to Sweatt, "Achromatic triplet using holographic optical elements" which appeared in Applied Optics, Vol. 16, No. 5, May 1977, pages 1390 and 1391. However, an optical system using only holographic elements would have the distinct disadvantage of being useful only over a small spectral band, i.e., the system would lack color correction. This lack of color correction would greatly limit the overall usefulness of a holographic system. The limitation to a small spectral band is due to the low and high order chromatic aberrations inherent in thin-lens equivalent holographic elements, i.e., holographic elements are more dispersive than conventional glass lenses.